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Pitfalls and Considerations: Seismic Surveys in Igneous and Metamorphic Environments for Geothermal and Mineral Resources

Smith, David A.*1; Hild, Jim 1; Pfeiffer, Jim 1; Drakos, Peter 2
(1) Technology, Zapata Incorporated, Charlotte, NC.
(2) Ormat Technologies Inc., Reno, NV.

Geothermal and mineral resources occur mainly in association with igneous and metamorphic geology, even in primarily sedimentary settings. In sedimentary settings, there is often a transition to a metamorphic regime caused by hydrothermal solutions from geothermal cells. Sites associated with igneous and metamorphic rocks are notorious for seismic signal attenuation and scattering. Additionally, the reflection events in igneous/metamorphic settings can result from impedance contrasts along dikes, alteration zones, density changes and numerous features other than lithologic boundaries. Given these factors, continuous reflections across stacked sections at these sites are seldom apparent. Rather, acoustic interfaces will typically manifest themselves as a series of short, discontinuous reflection events within a background of scattered acoustic energy. The resulting data sets are not easy to process and interpret.

Seismic reflection techniques will usually image the interface between sedimentary and crystalline rock very easily due to the high acoustic impedance contrast at this boundary. The objective of seismic surveys conducted for geothermal and mineral exploration is usually to gain some understanding of structural features within igneous and metamorphic rocks, which may include faults, fracturing, lithologic changes, etc. In order to successfully image in this setting, special considerations need to be addressed in data acquisition, processing and interpretation. This paper discusses survey design considerations for 2-D data in these environments, dealing with coherent source generated noise trains, and identification of reflection events through an examination of intermediate processing steps.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California